084 
FXUS65 KPSR 130444
AFDPSR
AZZ020>028-CAZ030>033-131045-

SOUTHWEST ARIZONA/SOUTHEAST CALIFORNIA FORECAST DISCUSSION 
NATIONAL WEATHER SERVICE PHOENIX AZ 
945 PM MST WED FEB 12 2003

.SYNOPSIS...
ABUNDANT MOISTURE ASSOCIATED WITH AN UPPER LEVEL LOW PRESSURE SYSTEM 
WILL SPREAD SHOWERS ACROSS SOUTHWEST ARIZONA AND SOUTHEAST 
CALIFORNIA TONIGHT AND THURSDAY. LOCALLY HEAVY RAINFALL CAN BE 
EXPECTED DURING THIS TIME. LINGERING SHOWERS WILL CONTINUE FRIDAY 
BEFORE HIGH PRESSURE MOVES IN FOR THE WEEKEND AND GIVES DRIER AND 
WARMER WEATHER INTO THE EARLY PART OF NEXT WEEK. 

.DISCUSSION...
UPPER CLOSED LOW THAT WAS OFF THE SRN CA COAST HAS OPENED UP...AND 
IS IN THE SLOW PROCESS OF MOVING EAST AND INLAND OVER THE DESERT SW. 
SLY FLOW AHEAD OF THE LOW HAS CREATED A SUBSTANTIAL TROPICAL TAP AND 
IS INGESTING COPIOUS AMOUNTS OF MOISTURE INTO SERN CA AND AZ. GOES 
PWAT IMAGERY SHOWS BETWEEN 1.5 AND 2 INCHES OF WATER JUST OFF CNTRL 
BAJA...AND LOCAL RAOBS ARE NEARLY SATURATED BELOW 500MB...WITH PWAT 
AT OR OVER 1 INCH. SCT TO NUMEROUS SHOWERS HAVE DEVELOPED CWA 
WIDE...BUT ARE FOCUSED EAST OF PHX AS OF 9 PM...AND FOR THE MOST 
PART THEY HAVE BEEN LIGHT THIS EVE. HOWEVER THERE ARE POCKETS OF 
MODERATE SHOWERS EMBEDDED IN THE OVERALL PRECIP SHIELD...AND 
RAINFALL IS EXPECTED TO CONTINUE TO INCREASE IN COVERAGE AND 
INTENSITY AS THE NIGHT WEARS ON. SURFACE DEWPOINTS HAVE RISEN 
DRAMATICALLY AS THE SHOWERS MOISTEN THE LOWER AMS...WITH RISES OF 20 
TO OVER 30 DEGREES IN THE PAST 24 HOURS. VALUES OVER THE DESERTS NOW 
ARE IN THE 50S TO LOW 60S. A LOOK AT IR IMAGERY SHOWS A SERIES OF 
RIPPLES MOVING UP FROM THE SOUTH...EACH OF THESE WAVES WILL ADD 
ENHANCED UVV TO THE ATMOSPHERE AS THEY PASS THRU THE AREA OVER THE 
NEXT 24-48 HOURS...AND WITH THE AIRMASS SO WET THEY WILL HELP SPUR 
LOCALLY HEAVY SHOWERS. LATEST ETA CONTINUES THE IDEA OF SLOWLY 
MOVING THE TROF EAST DURING THE NEXT 2 DAYS...WITH A SERIES OF VORT 
MAXES/SHORT WAVES MOVING THRU THE TROF AND ACROSS THE CWA. LATEST 
ETA AGAIN OVERDOING THE FORCING/UVV AND QPF VALUES...IF IT WAS 
CORRECT BY 06Z MUCH OF THE WRN CWA SHOULD HAVE PICKED UP OVER ONE 
HALF INCH OF RAIN....WHEREAS AS OF 9 PM MOST PLACES HAVE PICKED UP 
LESS THAN 2 TENTHS. ETA SUGGESTS COPIOUS RAINFALL WILL OCCUR OVER 
SERN CA AND SWRN AZ TONIGHT...WIHT UVVS OF OVER 10 MICROBARS/SEC 
...THEN THE DYNAMICS WILL SHIFT EAST AND MAINLY AFFECT THE AZ 
PORTION OF THE CWA THU INTO FRI. LATEST FOUS GUIDANCE FROM THE ETA 
GIVES AROUND 2.5 INCHES OF QPF FOR PHOENIX OVER THE NEXT 48 HOURS. 
THIS LIKELY OVERDONE...BUT EVEN IF HALF OF THIS OCCURS IT WILL BE A 
SIGNIFICANT EVENT FOR OUR AREA.

IN ANY CASE...FOR THE REST OF TONIGHT RAIN WILL BE ON THE INCREASE 
EVERYWHERE...WITH THE HEAVIEST AMOUNTS LIKELY TO FALL WELL AFTER 
MIDNIGHT. HAVE REMOVED THUNDERSTORMS FROM THE FORECAST TONIGHT AS 
AMS TOO STABLE...AND NO LIGHTNING HAS BEEN SEEN SO FAR ON THIS 
SHIFT. FOR THE MOST PART THE PRECIP HAS BEEN STRATIFORM...WITH NOT 
MUCH CONVECTION NOTED. AS THE TROF PUSHES FURTHER EAST THU AND SOME 
COLDER AIR ALOFT MOVES IN FROM THE WEST...AIRMASS SHOULD START TO 
DESTABILIZE...AND THERE WILL BE AN INCREASING THREAT FOR STORMS. 
THIS IS ALREADY IN THE FORECAST. BY THU AFTN MOST OF THE HEAVY 
PRECIP SHOULD BE CONFINED TO AZ...WITH LESSER AMOUNTS IN SERN 
CA...AND THE OVERALL FOCUS AREA WILL LIKELY SHIFT TO MARICOPA COUNTY 
EAST BY FRI...WITH DECREASING THREATS FOR SERN CA AND THE FAR 
WESTERN DESERTS AS RH STARTS TO DECREASE AND UVVS LESSEN. AGAIN THIS 
TREND COVERED NICELY IN THE CURRENT PACKAGE. HAVE MADE SOME MINOR 
TWEAKS TO THE FORECAST WORDING FOR TONIGHT AND THURSDAY...MAINLY 
REMOVING THUNDERSTORMS...OTHERWISE NO BIG CHANGES NEEDED. WITH 
AIRMASS SO WET AND STRONG FORCING STILL EXPECTED OVER AZ LATER 
TONIGHT THRU THU...COULD SEE SOME FLOODING PROBLEMS AND THUS WILL 
LET THE FLASH FLOOD WATCH CONTINUE IN EFFECT.  11

.PSR...FLASH FLOOD WATCH FOR SOUTH-CENTRAL AND SOUTHWEST 
ARIZONA...ZONES 21 THROUGH 28...FROM MIDNIGHT THROUGH 5 PM THURSDAY.

$$